Your headache might have a fairly innocuous cause—say, your neighbor’s drum lessons—but if you do a Web search for “headache,” you’ll discover that headache is a symptom of some quite serious but rare diseases, such as brain tumors. Are you tempted to conclude that you have a brain tumor?

A study released by Microsoft in January found that more than 90 percent of participants had looked at information about a serious disease on the Internet. Unfortunately, the number of Web search results available on a particular illness is not related to the frequency that the illness actually appears in the population. Health.com notes that although you can usually modify a search based on your particular symptoms, a search engine is still unable to make judgment calls based on your medical history and other external factors the way a doctor would.

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The Microsoft study acknowledged that plenty of useful medical information is available on the Web. The problem arises when people who are expert Internet searchers—and not medical experts—start using the Web to diagnose themselves with a variety of conditions. Many such searchers develop crippling anxiety and obsessive behaviors after determining they are sick based on Web information. The Microsoft researchers dubbed this condition “cyberchondria.”

A Slate magazine reporter connected this phenomenon to sufferers from benign fasciculation syndrome, which involves harmless muscle twitching. Unfortunately, many people who have experienced such twitching have researched their symptoms on the Web and decided they had Lou Gehrig’s disease (ALS), a neurodegenerative disease. Although the patients know they have the less harmful illness, many cannot stop being fearful.

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A study conducted by the Pew Internet & American Life Project in 2006 found that 80 percent of American Internet users had searched for health information on the Web.

Among this group of health information seekers, only about 25 percent checked their sources for reliability. As a result, many ended up feeling bewildered or uncertain about the information they found online. Yet many Americans are still relying on these untested sources to provide the bulk of their medical information.

It might seem odd that people would use potentially unreliable health information to diagnose themselves with rare and serious diseases, but there is an explanation: availability bias or the availability heuristic.

A theory of heuristics, or rules of thumb, developed in the 1970s, the availability heuristic states that people are more likely to believe something is true if it’s easier to imagine. Grave illnesses tend to inspire more vivid mental pictures than mundane ailments, and the sensational nature of devastating diseases inspires more people to write about them on the Web. Thus, once an Internet user pulls up several resources on ALS, she will more readily find it plausible that she has the disease.

The availability heuristic may be a factor in what is colloquially known as “medical school syndrome.” As soon as medical students learn how to diagnosis illness, many start believing that they are ill with the various diseases they study, Roger Collier writes in the Canadian Medical Association Journal. Although most of the students are physically well, the obsessive concern is a legitimate malady on its own, and doctors recommend that students discuss their anxieties with a doctor.

It’s important to remember that the problem is not with the Internet: It's with patients who don't check the reliability of sources, or succumb to availability bias before consulting with a doctor. Mark Moran, findingDulcinea founder and CEO, emphasizes on his blog Founding Dulcinea that the point of Internet research is to clarify what your doctor is telling you and help you ask intelligent, well-researched questions. The Internet can't make you smarter than your doctor, or allow you to avoid seeing a doctor, or justify ignoring your doctor's advice.

When searching for health information, the most important thing to remember is that not all Web sites are created equal. If you’re punching your symptoms into a computer, don’t rely on a regular search engine; rather, choose reliable, trustworthy health Web sites. Guidelines and recommendations for health research on the Internet can be found in findingDulcinea's Web Guide to Health.